1. Field of the Invention
The present invention relates to a method and apparatus for controlling a power transmitting system for an automotive vehicle, which includes a continuously variable transmission and an auxiliary transmission connected to the continuously variable transmission.
2. Discussion of the Related Art
A motor vehicle is known, which has a power transmitting system which includes a continuously variable transmission connected to an auxiliary transmission which has two or more forward drive gear positions. In this type of power transmitting system, the auxiliary transmission is automatically shifted from one of the forward drive positions to the other, depending upon the varying running conditions of the vehicle, for example, an opening angle of the throttle valve and a running speed of the vehicle. Further, the speed ratio of the continuously variable transmission is controlled so that the actual speed of the input shaft of the continuously variable transmission coincides with a target speed which is determined based on a currently required output of the engine and/or other variable or variables, according to a predetermined relationship between the target speed and the variable or variables. The predetermined relationship for determining the target speed of the input shaft of the continuously variable transmission for controlling its speed ratio, is used for all operating conditions of the vehicle, irrespective of the currently selected drive position of the auxiliary transmission, and/or the currently selected position of a shift lever or other selector device for selecting the operating mode of the auxiliary transmission. Such a predetermined relationship that should be used for determining the target speed is basically the same for all forward drive positions of the auxiliary transmission. Precisely, however, the relationship should be changed depending upon the selected forward drive position of the auxiliary transmission, since the range of variation in the overall speed ratio of the power transmitting system is varied depending upon the selected drive position of the auxiliary transmission.
When the auxiliary transmission of a power transmitting system of the type indicated above is shifted down from one drive position to another drive position of a lower speed range, the actual speed of the engine, or the actual speed of the input shaft of the continuously variable transmission is raised due to a difference between the speed ratios of the auxiliary transmission in the two drive positions of different speed ranges. However, the engine speed should not exceed its maximum permissible speed Ne.sub.max. Therefore, the engine speed immediately before a shift-down operation of the auxiliary transmission must be lower than a predetermined upper limit n1 (n1=Ne.sub.max /r, if the speed ratio of a high-range position of the auxiliary transmission is 1, and where r represents the speed ratio of a low-range position of the auxiliary transmission). This means that the vehicle speed at which the auxiliary transmission is permitted to be shifted down must be relatively low, to avoid an excessive rise of the engine speed when the auxiliary transmission is shifted down. Accordingly, the vehicle tends to suffer from a relatively low level of drivability.
The conventional power transmitting system discussed above also suffers from inconveniences associated with a shift-down operation of the auxiliary transmission while a shift lever is placed in a low-speed or engine brake position. This position is provided to apply an engine brake to the vehicle or to obtain a boosted drive force. In this engine brake mode of the conventional system, the same relationship for determining the target speed of the input shaft of the continuously variable transmission is used before and after the auxiliary transmission is shifted from its high-range position to its low-range position. This does not permit a shift-down of the auxiliary transmission to provide an effective engine brake or a sufficiently large drive force to the vehicle. More particularly, the relationship used for determining the target speed must be prepared such that the engine speed will not rise beyond its maximum permissible speed Ne.sub.max, immediately after the auxiliary transmission is shifted down. To this end, the relationship is determined so that the shift-down operation of the auxiliary transmission is initiated only after the vehicle speed has been lowered down to an upper limit at which the engine speed will exceed its maximum permissible level Ne.sub.max immediately after the auxiliary transmission has been shifted down to its low-range position. This also means that the engine speed immediately before the shift-down operation must be lower than a certain limit. The shift-down operation at relatively low vehicle speed and engine speed will not provide a sufficient engine brake, or a satisfactory level of drivability.